Mask Design Using Existing Object Catalogues

Using dedicated GMOS images (pre-imaging) for designing MOS masks has the advantages of being independent of the accuracy of any astrometric calibration and immune against proper motions of alignment stars or objects. However it also requires more telescope time, and requires time between the start of the semester and the start of the MOS observations for the pre-imaging to be taken and the mask to be designed. If targets and alignment stars can be selected from the same object catalogue which has accurate enough relative astrometry, the MOS masks can be created without GMOS pre-imaging.

The steps to create an Object Table using an object catalog are:

Use the on-sky test results to evaluate if the expected centering accuracy and consequent slit losses are acceptable. For most observations with narrow slits (< 1.0 arcsec) or very long integrations (> 10 hours) one may wish to consider using GMOS pre-imaging instead.

Run the gemini IRAF task gmskcreate to create the
Object Table and a pseudo-GMOS image

The Object Table and the pseudo-GMOS image are then used as inputs to GMMPS and from that
point on the process of mask creation is the same whether an object catalog or pre-imaging
was used.

On-sky test results

The transformations between RA and Dec and GMOS x,y were found by observing fields taken from the UCAC2 catalog and noting the displacement between the slit centers and the observed star position. These astrometric fields contain several tens of stars but only those with position errors <20 mas were accepted for the design of test masks. The slits in the test masks were 2"x2" squares to allow the slit centers and the star centroids to be accurately measured. The errors on the astrometry of stars used to derive the transformations are ≈30 mas (South) and ≈60 mas (North).

The two plots below show the measured displacements, where green and red symbols indicate results from two different masks rotated by 90 degrees. The maximum displacements are less than 0.15" in North and South and the r.m.s. of the displacements are 0.06" which is less than a single unbinned GMOS pixel.

The displacement of an object within a slit translates into loss of flux, with the
fraction lost depending on the selected slit width and seeing. The plot below illustrates a
simple case of a point source (Gaussian profile) observed through slits with the width
matched to the seeing (i.e. 0.5" slit used in 0.5" seeing FWHM) with an extraction
aperture of 1.4*FWHM. The color curves indicate the fraction of flux in the aperture as
compared to a perfectly centered target for different slit widths. The solid vertical line
indicates the maximum displacement measured for the test masks, which can be considered as a
conservative estimate of slit losses as most objects on a mask would be displaced by a
smaller amount.

The losses should be viewed in terms of the telescope time
saving achieved by abandoning the pre-imaging. The plot below
indicates the integration time for which the time needed to cover the
additional slit losses is still less than the time needed for
pre-imaging. It assumes the very minimum time needed for pre-imaging
(currently 15 min).

For example, in the above plot, for a slit 1 arcsec wide the break-even exposure time is
~6.5 hours. For all exposure times less than this, adding 15 mins to the exposure time would
compensate for the expected slit losses.

The above plots can be used to estimate whether an object catalog
is suitable to construct the mask. If the relative accuracy of the
astrometric solution for the catalog is similar to the accuracies
quoted for the tests, then the displacements can also be expected to
be similar. The object catalog must include both your science targets
and the alignment stars, with homogeneous astrometry from single solution. In
most cases the astrometric accuracy should be better than 0.1"
rms. However, for shorter exposures poorer astrometry could still
produce an overall time gain. Another point worth consideration is the
logistical advantage of being able to design the masks even before the
target becomes observable.

Running gmskcreate

The Object Table and pseudo-GMOS image are created using the IRAF task
gmskcreate that is part of the Gemini IRAF mostools package.

gmskcreate requires an input file containing information about the
spectroscopy candidates, the Gemini program ID and the RA, Dec and
position angle of the required Gemini field. Note that this RA, Dec
and position angle must be identical to that defined in the Phase-II
observations, and the PI must check before submitting the mask that
there is a suitable guide star available.

gmskcreate can create an Object Table and/or a pseudo-GMOS image. The
pseudo-GMOS image is required for the mask checking but does not have to be of high quality.
It is used to verify certain properties of the mask (eg. to make sure the acquisition
objects do not fall in the gaps between the CCDs) or to check for gross slit placement
problems. It is expected that in most cases the mask will be designed using the previously
determined astrometry and not by measuring the target pixel positions in the pseudo-GMOS
image.

The input data file

The input data file must contain one line per object. Each line
must contain values for 'ID', 'RA', 'DEC', 'MAG' in that order
and separated by spaces, where 'ID' is an integer ID number for the
object, 'RA' and 'DEC' are the Right Ascension and Declination of the
object, and 'MAG' is the magnitude of the object.

The line can also optionally contain values for 'priority', 'slitsize_x', 'slitsize_y', 'slittilt' and 'slitpos_y', again in
that order and separated by spaces. These values are used by (and can be altered in) GMMPS. 'Priority' is a single character priority
("0/1/2/3/X") for the object (0 is for acquisition objects, 1-3 for targets, 1 is highest, X is for objects to be ignored), 'slitsize_x' is the slit width (real) in
arcsecs, 'slitsize_y' is the slit length (real) in arcsecs, 'slittilt' is the position angle of the slit in degrees measured
counter-clockwise relative to the default position, and 'slitpos_y' is the slit position in the y-direction relative to the
object position (real). It is possible to set the optional values for just a few of the objects in the file, and not all of
the optional values must be set. If for any object only a few (or none) of the optional values are set the remaining ones will
inherit the default values of priority=1, slittilt=0, slitpos_y=0, and slitsize_x and slitsize_y as given in the input
parameters slitszx and slitszy. Note that acquisition objects (priority = 0) must have slitsize_x = 2.0, slitsize_y = 2.0,
slittilt = 0 and slitpos_y = 0 so any optional values given for acquisition objects will be ignored.

Please note the following limitations in gmskcreate:

There is no handling of epoch or equinox information within gmskcreate. The user must ensure that the RA and
Dec coordinates in the input data file match the epoch and equinox of the WCS in the input image used to create the pseudo-GMOS
image.

Related to the equinox is the issue of proper motion, there is no automatic handling of proper motions in either gmskcreate
or GMMPS. Users are advised to avoid using high proper motion stars as alignment objects, and if any slits are place on high
proper motion stars those input coordinates must be properly corrected to reflect valid positions during the semester in which
the mask is to be observed.

In the input table RA and Dec coordinates must be given in decimal form, hms and dms (hh:mm:ss.ss and dd:mm:ss.ss)
sexagesimal formats are not supported.

Some of these limitations may be addressed in a future release of the
gmskcreate task.

Creating the Object Table

gmskcreate will create the Object Table if fl_getxy=yes.

The Object Table is a FITS table that will contain
the following columns:

ID

Unique object id (integer)

RA

RA in hours (real)

DEC

Dec in degrees (real)

x_ccd

X coordinate of object position in pixels
(real)

y_ccd

Y coordinate of object position in pixels
(real)

MAG

(Relative) magnitude of object (real)

Priority

Priority of object (char*1; "0/1/2/3/X")

slitsize_x

Slit width in arcsec (real)

slitsize_y

Slit length in arcsec (real)

slittilt

Slit position angle in degrees (real)

slitpos_y

Slit position in the Y-direction (spatial) relative to the object
position in arcsec (real)

The GMOS x,y pixel values are calculated from the input RA and Dec using the
known transformations.
It is crucial
that the RA and Dec for all objects on the mask (spectroscopy candidates
and alignment stars) comes from the same astrometric solution.

The Object Table name is given in the input parameter outtab.The
file outtab will be given the extension '.fits' if this is not already
included in the name. If fl_getxy=yes and outtab is undefined then
the Object Table name will be the indata file name with the prefix
'GMI' and the suffix '_OT.fits'.

The task will also create the file given in the outcoords parameter
which is an ascii file containing the GMOS x,y coordinates, one object
per line. This file is not required for GMMPS but may be useful for
overplotting the GMOS x,y positions on the pseudo-GMOS image using for
example the IRAF task tvmark. If fl_getxy=yes and outcoords is
undefined then the output file will be the indata file name with the
prefix 'GMI'.

Creating the pseudo-GMOS image

gmskcreate will create the pseudo-GMOS image if fl_getim=yes.

The transformation from input image to pseudo-GMOS image is found by
first using the input image WCS header keywords to transform the RA
and Dec of an evenly distributed grid of 25 positions throughout a 5.5' x 5.5' field of view
into input-image x,y positions, and then
calculating the transformation between the input-image x,y positions
and the GMOS x,y positions (calculated from the RA and Dec and the
known transformations).

The transformation from input x,y -> GMOS x,y is calculated using
geomap with a polynomial of order 2:

xgmos = a + b*xinp + c*yinp ; ygmos = d + e*xinp + f*yinp

being used for the default fit (allowing for shift, scale and
rotation).

This procedure requires that the input image contains the WCS keywords,
CRPIX1, CRPIX2, CRVAL1, CRVAL2, CD1_1, CD1_2, CD2_1 and CD2_2. For DSS
images these keywords can be added with the iraf command makewcs. The
above procedure assumes that the WCS in the input image corresponds
reasonably well with the astrometry of the input objects. If this is
not the case the user may want to update the WCS of the input image.

If the WCS of the input image and the astrometry of the input
objects do not correspond well, this will cause the slits plotted by
GMMPS to not lie on top of the objects. This is not a problem for the
mask creation, as the GMOS x,y positions are taken from the Object Table
and not from the image, but it may make it more difficult to check the
mask.

The pseudo-GMOS image is given the name in the outimage parameter
with the prefix 'GMI' added. The prefix is necessary to identify images as
pseudo-GMOS images when they are submitted to the observatory using
the Observing Tool. If outimage is undefined then the output image
name will be created from the inimage name with the prefix GMI.

An example

Create a Object Table and pseudo-GMOS image for an observation to be
taken with GMOS-S.

In this case the user has set the priority, slitsize_x, slitsize_y, slittilt and slitpos_y for the first and third objects
in the file (id = 10 and 12). The priority has also been changed from the default value priority = 1 to priority = 3 for object id = 14. The priority, slitsize_y
and slittilt has been changed for object id = 18. Note that if you do not want to specify all of the optional values you still must specify all the values preceeding
the last one whose value you wish to change for an individual object id.

Objects id = 17, 20 and 21 are examples of acquisition objects where priority is set to 0, the slitsize_x, slitsize_y, slittilt and slitpos_y values will automatically default to values of 2.0, 2.0, 0 and
0 even if the user had specified other values for the optional parameters.
The aquisition objects can be selected interactively in GMMPS, however, it is important for the object table to contain enough appropriate sources. Having additional acquisition objects to choose from
will allow more flexibility during the design in GMMPS.

Images: GMMPS requires that certain keywords are in the pseudo-GMOS
image header. The gmskcreate script adds these required keywords to
the header. GMMPS has been run successfully with images from the
following sources. Note that this does not test the astrometric
accuracy of the masks, just that the mask software is able to create
the masks when the input images come from these sources. Please report any problems with using images from other
sources via the Gemini Helpdesk under the topic Gemini IRAF.

Tested with images from:

DSS (run makewcs if the WCS image header keywords are not present)

Sloan DR5

CTIO MOSAIC

HST ACS

CFHT MegaPrime

HST WFPC (run wmosaic and work with the mosaic'd image)

Recommendations to ensure a Good Mask Design when using Object Catalogs

Because there is
a greater potential for failure, PIs must understand that they use the capability to design MOS masks from object catalogs at their own risk. We offer the following
suggestions to minimize the chances for losing significant amounts of observing time due to a poorly designed mask:

Make sure that your OIWFS guide star is not also one of your mask alignment objects, the same star cannot be
used for both purposes.

We require that masks made from catalogs have at least three acquisition objects and they must be distributed across
the field (this is actually a good practice for all MOS masks).

We suggest that when observing faint targets in their MOS mask, the PI insert 2-3 bright stars into their object table.
These stars need to be bright enough so that a spectrum can be easily detected in one science exposure. These stars should be
distributed across the field of view in order to best sample the accuracy of the slit placement. (PIs should note that they
can use this to a scientific advantage if they select stars from which a useful telluric calibration may result. This is best
achieved by selecting one star at the right side of the field of view and the other at the left in order to cover the whole
wavelength range of the mask).

If the acquisition does not give an acceptable alignment the data will not be taken. This is the same requirement as
imposed on masks designed from GMOS images. An acceptable alignment means the rms offset of the acquisition objects from the
center of the acquisition boxes is less than 25% of the slit width of the narrowest science slits.

The Gemini Observer will note that IF the mask has been designed with 2 or 3 bright stars in it (this needs to be made
clear in a NOTE submitted via the OT when the mask design is submitted and the MOS observations are finalized) they will
inspect the first science exposure and IF there are no obvious spectra from these bright stars, they will STOP taking data.
The PI will be asked to retrieve the data from the archive and inspect the spectra, and provide feedback as to whether or not
they wish the observations to continue or if they want to design a new mask (perhaps using pre-imaging to do so).

IF obvious spectra are seen from these bright stars (or from science targets which are bright enough to produce a spectrum
in a single exposure) the observing will continue.

IF no bright stars are placed in the mask and if the science targets are too faint to be seen in a single exposure the data
will be taken and if the PI determines after the fact that the mask was not good and the slits were not aligned on the science
targets then they must reapply for time in order to obtain more data in a subsequent semester. Gemini will not re-take these
data for the PI and the time will be charged to the program and the partner.